The nation's sewer system is aging and that means it is also wearing out with the risk of broken pipes leaking raw sewage into streets and living rooms.

Mark T. Hernandez and colleagues writing in Environmental Science&Technology note that the maintenance of U.S. wastewater collection systems costs an estimated $4.5 billion every year, much of which goes toward fixing or replacing 8,000 miles of sewers, and project that n the future, these annual costs could top $12 billion.

Part of the problem is corrosion caused by sewer gases that feed acid-generating microbes, which grown in biofilms on the inside top surface of the pipes. Although microbe communities have long been recognized as a factor in the corrosion of concrete pipes, they have not been well studied. To fill in the gaps, Hernandez's team decided to figure out what kinds of bacteria and other conditions were most closely associated with corrosion problems.

They found that there are certain conditions in the pipes can clue utilities in to which ones need repair before it's too late. Pipe failure can be predicted.

From 10 different sewer systems in major cities around the country, the scientists measured bacterial diversity, gas concentrations in the air above the wastewater and other factors. In the most worn pipes, they found markedly little variety in the kinds of bacteria present, as well as elevated levels of both hydrogen sulfide and carbon dioxide gases.

The researchers concluded that wastewater utilities could economically monitor combinations of these gases in sewage pipes to figure out which sites might be at higher risk for corrosion and take the necessary steps to prevent major damage.